Telecommunications system

ABSTRACT

In a TDM communications system comprising a base station and a number of outstations, a marshalling signal enables a newly connected outstation to determine the timing of its transmission bursts A detector circuit for recovering the marshalling signal incorporates comparators whose thresholds are adjusted to the high and low levels of the normal data signal. The marshalling signal is recovered from one or other of the comparators according to whether a high or low normal data signal is received, this condition being determined by a third comparator whose threshold is adjusted midway between the high and low level. This allows the marshalling signal to be recovered continuously without interrupting normal data transmission from the outstation.

This invention relates to telecommunications systems and in particularto systems employing the time division multiplex/time division multipleaccess (TDM/TDMA) principle.

BACKGROUND OF THE INVENTION

The TDM/TDMA principle is well known in radio systems or passive opticalnetworks (PONs), where it is employed to permit transmission between asingle base station and a plurality of outstations. In the downstream(base station to outstation) direction, the information (traffic) isbroadcast to all outstations, but upstream it is transmitted in bursts,each of which must be timed to avoid mutual interference (overlap) sothat at any time the base station only receives data from oneoutstation. When a new outstation is to be connected its time oftransmission must be such that it does not interfere with existingtraffic transmissions and the processing required to ensure this isreferred to as marshalling.

A known method of marshalling involves reservation of a period of notransmission in the upstream frame (a window). It will be appreciatedthat the base station broadcasts information to all outstationsemploying a frame, which is typically of length 125 microseconds. Theoutstations are all able to receive the downstream traffic at adifferent time, depending on their distance from the base station andthe propagation delay.

Typically, in the steady state, several outstations may be alreadyconfigured to send their data burst to the base station at a time whichwill ensure their arrival in separate bursts. A new outstation will,according to the known marshalling method, make a trial transmissioninto the window. The position of this is subsequently detected at thebase station and used to determine the marshalling delay. In this knownapproach, the window in the upstream traffic flow must be at least aslarge as the uncertainty in the downstream and upstream loop delay. Forexample, with a PON, the loop delay may be 200 microseconds for a 20 Kmrange (i.e. the outstations may be up to 20 Km from the base station),requiring a 200 microsecond window, which would result in an equivalentadditional delay in the upstream traffic, the need for all operatingoutstations to queue for one window's duration traffic and necessitatessubsequent storage facilities, and a loss of bandwidth given by theproduct of the frequency of the window and its size in bits.

Our published United Kingdom specification No.2272610 describes a methodof marshalling an outstation of a TDMA telecommunications system, thesystem including a base station and a plurality of outstations. Themethod includes transmitting from the outstation a sequence at a levelbelow the noise sensitivity of a receiver at the base station, detectingthe sequence at the base station, discriminating the phase of thedetected sequence, and using the discriminated phase to determine theloop delay to the outstation. The sequence and its phase are determinedby a correlation process.

It is an object of the present invention to provide an improvedmarshalling process.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anarrangement for marshalling an outstation in a time division multipleaccess (TDMA) telecommunications system including a base station and aplurality of outstations, each said outstation having means fortransmitting a marshalling sequence continuously and in parallel with atransmitted data signal, said data signal having first and second levelconditions, wherein the base station incorporates first means fordetecting the marshalling signal when the corresponding data signal isin its first level condition, second means for detecting the marshallingsignal when the data signal is in its second level condition,charaterised in that the base station incorporates means (29) responsiveto the level condition of the data signal for selectively enabling thefirst or second detector means.

According to a further aspect of the invention there is provided amethod of marshalling an outstation in a time division multiple access(TDMA) telecommunications system including a base station and aplurality of outstations, the method comprising transmitting from eachsaid outstation a marshalling sequence continuously and in parallel witha transmitted data signal, said data signal having first and secondlevel conditions, characterised in that the level condition of the datasignal is determined at the base station, that the marshalling signal isdetected at the base station via first detecting means when thecorresponding data signal is determined to be in its first levelcondition, and that the marshalling signal is detected at the basestation via second detecting means when the data signal is determined tobe in its second level condition.

As the marshalling or correlation signal is received continuously, therequirement, inherent in previous systems, to cease normal transmissionperiodically to recover the marshalling signal is avoided. This resultsin an improvement in system capacity and in the quality of service.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of a TDMA network;

FIG. 2 is a circuit diagram of a receiver arrangement for the network ofFIG. 1 for recovering both data and marshalling signals; and

FIG. 3 illustrates the composite received signal wave forms of thecircuit of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, the network, which is depicted in highlyschematic form, comprises a base station 11 and a number of outstations12. The base station incorporates a transmitter 110 and a receiver 111whereby two way communication may be established via correspondingreceivers 120 and transmitters 121 provided in each outstation.Communications may be effected in a TDMA format wherein each outstation12 wishing to transmit is allocated one or more time slots, thisallocation being known as marshalling.

The system of FIG. 1 may comprise e.g. a passive optical network (PON)or a cellular radio network.

The marshalling of a new outstation is achieved by the transmission of alow level data sequence which is sent at a level which is below thatwhich degrades the ability of the base station detector to correctlyreceive the traffic being received from outstations in the steady state.The signal to noise ratio required by a detector for a bit error ratioless than 10⁻⁹ is approximately 11 dB (optical), consequently, anoutstation to be marshalled must transmit at a level such that theincident power of that marshalling signal at the base station receiveris at least 11 dB lower than the weakest expected steady outstation.Recovery of the marshalling signal and the data signal received from anoutstation is effected via a receiver circuit 112. This circuit will bedescribed in detail below.

Referring now to FIGS. 2 and 3, a composite wave form including data andmarshalling or correlation signals is received at the circuit input

(FIG. 2) which typically comprises a photo diode D1. This input signal,which is treated as an analogue signal, is compared with three differentanalogue reference levels respectively by three comparators 21, 31 and41 whereby to generate corresponding digital signals. During operationof the receiver circuit, the reference or threshold levels of thecomparators are continuously monitored and adjusted. These thresholdlevels are defined below.

High Threshold

This is the level supplied to comparator 21 and approximates to the meanlevel of the analogue signal when the normal data signal has the valueONE.

Low Threshold

This is the level supplied to comparator 41 and approximates to the meanlevel of the analogue signal when the normal data signal has the valueZERO.

Normal Data Threshold

This is the level supplied to comparator 31 and approximates to the meanof the High and Low thresholds.

The High and Low thresholds are derived from respective digital toanalogue converters (DAC's) 25 and 26. The Normal Data threshold isderived as the mean of the High and Low thresholds via a potentialdivider comprising two substantially equal resistors R1 and R2. TheDAC's 25 and 26 which generate the High and Low thresholds are drivenfrom respective digital counter circuits 27 and 28.

The counting up and down of the two counters 27 and 28 in response tothe outputs of the comparators 25 and 26 provides for automatic trackingof the optimal High and Low thresholds. The counters derive their timingfrom a digital clock signal CLK. On each clock edge, one or other of thecounters will count in the appropriate direction to follow the receivedlogic levels. However, it is advantageous to apply low pass filtering tothe changes in the counter values in order that-the thresholds changerelatively slowly e.g. in response to variations in component andenvironmental parameters rather than in response to the marshallingsignal itself. This may be achieved by deriving the thresholds from themost significant bits of the counter outputs. For example, if themarshalling sequence is repeated every 32 clock periods, then the fiveleast significant bits of the counter outputs may be ignored.

The three comparator outputs are retimed to the digital clock CLK. Thenormal data traffic is obtained directly from the retimed output ofcomparator 21. The correlation or marshalling signal is obtained eitherfrom the retimed output of comparator 31 or of comparator 41, thisselection being performed by a multiplexer 29 in response to the outputof comparator 31.

The operation of the circuit will now be described with reference toFIGS. 2 and 3.

On the first active clock edge, the instantaneous value of the receivedsignal is below the Normal Data threshold. Therefore the data is assumedto have a value ZERO and the counter 27 controlling the High thresholdis not enabled and retains its current value (A--FIG. 3), while thecounter 28 controlling the Low threshold counts down because thereceived signal is below the Low threshold (B). The correlation ormarshalling signal is assumed to have the value ZERO. On the secondclock edge, the received signal has a value greater than the Normal Datathreshold so tale data signal is received as a ONE. The High thresholdis adjusted downwards (C) because the received signal level is belowthat threshold, while the Low threshold remains unchanged (D). At points(E) and (F) respectively of FIG. 3, the Low threshold is adjusteddownwards while the High threshold remains unchanged. At points (G) and(H) the High threshold is adjusted upwards twice in succession inresponse to an increase in the normal data signal level. Such dynamicchanges in amplitude are typical in a practical communications system.

The technique is applicable both to point-to-multipoint and topoint-to-point networks. The normal data transmitted from a plurality ofoutstations or remote nodes is received at the hub or base station nodeof such a network e.g. using a TDMA technique in which transmissions arereceived from each outstation in turn. To apply the technique to such anetwork, the values in the counters 27 and 28 are selected individuallyto match the transmission level of each outstation. This may be achievedby storing appropriate values for each outstation and, at any particulartime, retrieving the values for the particular outstation whosetransmissions are expected. In such an arrangement a guard band may beprovided between successive transmissions from outstations to allow timefor retrieval of the corresponding stored data. During such a guardband, or during any period within which no data transmission is beingreceived, the receiver circuit must be controlled to ensure that themarshalling signal is received correctly. For this purpose the "nosignal" input to gate 33 of the circuit of FIG. 2 is held at the logicONE level and the High threshold alone is used for signal detection.

In the circuit arrangement described above, the Normal Data threshold isset to the mid-point of the received value of the high and low logiclevels of the normal data signal irrespective of the proportion of ONE'sand ZERO's in that signal. For example, if the normal data contains tenlogic ONE's to each logic ZERO, then the High threshold will be adjustedten times more often than the Low threshold. However, the adjustmentsthemselves are not prejudiced by the frequency of their occurrence andthe mean level comprising the Normal Data threshold is thus unaffected.Setting this threshold to the mid-point level is advantageous as thisminimises the bit error rate (BER). The technique thus overcomes therequirement of conventional systems to balance the numbers of ONE's andZERO's.

I claim:
 1. An arrangement for marshalling an outstation in a timedivision multiple access (TDMA) telecommunications system including abase station and a plurality of outstations, each said outstation havingmeans for transmitting a marshalling sequence continuously and inparallel with a transmitted data signal, said data signal having a firsthigh logic level condition and a second low logic level condition,wherein the base station incorporates first detection means fordetecting the marshalling signal when the corresponding data signal isin its first level condition, second detection means for detecting themarshalling signal when the data signal is in its second levelcondition, wherein the base station incorporates means responsive to thelevel condition of the data signal for selectively enabling the first orsecond detector means, wherein said first and second detection meanscomprise respective first and second comparators each of whose thresholdis adjusted to the respective high or lower level condition of the datasignal so as to effect detection of the marshalling signal during thatdata signal level condition, and wherein the high or low level conditionof the data signal is determined via a third comparator whose thresholdis adjusted to a level between said first and second levels.
 2. Anarrangement as claimed in claim 1, wherein the respective thresholds ofthe first and second comparators are derived each from a correspondingcounter arranged to count clock pulses in an increasing or decreasingdirection corresponding to the current respective first or second levelcondition of the data signal.
 3. A method of marshalling an outstationin a time division multiple access (TDMA) telecommunications systemincluding a base station and a plurality of outstations, the methodcomprising transmitting from each said outstation a marshalling sequencecontinuously and in parallel with a transmitted data signal, said datasignal having a first high level condition and a second low levelcondition, determining at the base station the current high or low levelcondition of the data signal received from the outstation, detectingsaid marshalling signal at the base station via a first detection meanswhen said data signal is in its first condition and via a seconddetection means when said data signal is in its second condition, andcorrelating said detected marshalling signal so as to determine a timingreference for the outstation.
 4. A method of marshalling an outstationin a time division multiple access (TDMA) telecommunications systemincluding a base station and a plurality of outstations, the methodcomprising transmitting from each said outstation a marshalling sequencecomprising a pseudo-random binary sequence continuously and in parallelwith a transmitted data signal, said data signal having a first highlevel condition and a second low level condition, determining at thebase station the current high or low level condition of the data signalreceived from the outstation, detecting said marshalling signal at thebase station via a first detection means comprising a first comparatorwhose threshold level corresponds to the first level condition of thedata signal when said data signal is in that first condition, and via asecond detection means comprising a second comparator whose thresholdlevel corresponds to the second level condition of the data signal whensaid data signal is in that second condition, and correlating saiddetected marshalling signal so as to determine a timing reference forthe outstation.
 5. A method as claimed in claim 4, wherein the first orsecond level condition of the data signal is determined via a thirdcomparator whose threshold is adjusted to a level between said first andsecond levels.
 6. A method as claimed in claim 5, wherein the respectivethresholds of the first and second comparators are derived each from acorresponding counter arranged to count dock pulses in an increasing ordecreasing direction corresponding to the current respective first orsecond level condition of the data signal.